1. Field of the Invention
This invention relates to a method and apparatus for testing telecommunications apparatus.
2. Related Art
In testing telecommunications apparatus (for example, a telephone line, a telephone network, or communications apparatus such as a coder) a test signal is introduced to the input of the telecommunications apparatus, and some test is applied to the resulting output of the apparatus. It is known to derive "objective" test measurements, such as the signal to noise ratio, which can be calculated by automatic processing apparatus. It is also known to apply "subjective" tests, in which a human listener listens to the output of the telecommunications apparatus, and gives an opinion as to the quality of the output.
Some elements of telecommunications systems are linear. Accordingly, it is possible to apply simple artificial test signals, such as discrete frequency sine waves, swept sine signals or chirp signals, random or pseudo random noise signals, or impulses. The output signal can then be analyzed using, for example, Fast Fourier Transform (FET) or some other spectral analysis technique. One or more such simple test signals are sufficient to characterise the behaviour of a linear system.
On the other hand, modern telecommunications systems include an increasing number of elements which are nonlinear and/or time variant. For example, modern low bit-rate digital speech coders, forming part of mobile telephone systems, have a nonlinear response and automatic gain controls (AGCs), voice activity detectors (VADs) and associated voice switches, and burst errors contribute time variations to telecommunications systems of which they form part. Accordingly, it is increasingly less possible to use simple test methods developed for linear systems to derive objective measure of the distortion or acceptability of telecommunications apparatus.
On the other hand, subjective testing by using human listeners is expensive, time-consuming, difficult to perform, and inconsistent. However, despite these problems the low correlation between objective measures of system performance or distortion and the subjective response of a human user of the system means that such subjective testing remains the best way of testing telecommunications apparatus.
Recently in the paper "Measuring the Quality of Audio Devices" by John G. Beerends and Jan A. Stemerdink, presented at the 90th AES Convention, 1991 Feb. 19-22, Paris, printed in AES Preprints as Preprint 3070 (L-8) by the Audio Engineering Society, it has been proposed to measure the quality of a speech coder for digital mobile radio by using, as test signals; a database of real recorded speech and analyzing the corresponding output of the coder using a perceptual analysis method designed to correspond in some aspects to the processes which are thought to occur in the human ear.
It has also been proposed (for example in "Objective Measurement Method for Estimating Speech Quality of Low Bit Rate Speech Coding", Irii, Kurashima, Kitawaki and Itoh, NTT Review, Vol 3. No. 5 September 1991) to use an artificial voice signal (i.e. a signal which is similar in a spectral sense to the human voice, but which does not convey any intelligence) in conjunction with a conventional distortion analysis measure such as the cepstral distance (CD) measure, to measure the performance of telecommunications apparatus.
It would appear obvious, when testing apparatus such as a coder which is designed to encode human speech, and when employing an analysis method based on the human ear, to use real human speech samples as was proposed in the above paper by Beerends and Stemerdink. In fact, however, the performance of such test systems is not particularly good.